1. Field of the Invention
The present invention relates to a method for feedback and transmission scheme of multi-user (MU) multiple input multiple output (MIMO) in a wireless communication system. The method of the present invention provides processes for operating a base station in a multi-user MIMO configuration. The present invention also provides a base station that operates according to the feedback and transmission scheme of the present invention.
2. Description of the Related Art
A multiple antenna communication system, which is often referred to as multiple input multiple output (MIMO) system, is widely used in combination with orthogonal frequency division multiplexing (OFDM) technology in a wireless communication system to improve system performance.
A MIMO system uses multiple transmit antennas and multiple receive antennas to improve the capacity and reliability of a wireless communication channel. Therefore, the transmitter is capable of transmitting independent data streams simultaneously in the same frequency band. MIMO technology increases the spectral efficiency of a wireless communication system by exploiting an additional dimension of freedom in a space domain due to multiple antennas. A MIMO system promises linear increase in capacity with K where K is the minimum of number of transmit (M) and receive antennas (N) i.e. K=min(M,N). In a simplified example of a 4×4 MIMO system, four different data streams are transmitted separately from the four transmit antennas. The transmitted signals are received at the four receive antennas. Some form of spatial signal processing is performed on the received signals in order to recover the four data streams. An example of spatial signal processing is V-BLAST which uses the successive interference cancellation principle to recover the transmitted data streams. Other variants of MIMO schemes include schemes that perform some kind of space-time coding across the transmit antennas (e.g. D-BLAST) and also beamforming schemes such as spatial division multiple access (SDMA).
One of the disadvantages of the single-user MIMO PARC scheme is that multiple channel quality indicator (CQI) estimates are required for each of the individual streams. This results in excessive signaling overhead resulting in system inefficiency. In case of multi-user MIMO approach, it is possible to implement PARC scheme with just one CQI feedback per user. In this case, each user reports the best CQI determined by using, for example, a minimum mean square error (MMSE) algorithm along with the MIMO stream identity. However, multi-user MIMO requires that a large number of users are present in the system so that each user can be selected for transmission when it experiences best channel quality. However, when the number of users in the system is small, it is less likely to find users at their peak channel conditions and thus degrading the performance of a multi-user MIMO scheme. In other words, in a fixed configuration of a multi-user MIMO scheme, the performance of the multi-user MIMO system will degrade with few users. Therefore, it is necessary to provide a MIMO scheme that improves the performance of the MIMO system regardless of the number of users in a cell.